Determining relative timing offset in different electronic pathways using internal signals

ABSTRACT

A process and system including a detector having a photosensor therein that outputs a signal and a plurality of after-pulse detector devices independently connected to the photosensor via respective pathways. The after-pulse detector devices each detecting an after-pulse in the signal, where the after-pulse represents an after-event in the photosensor triggered from a previous photon generating event. The system further includes a processing device that receives an indication of the detection of the after-pulse from each of the plurality of after-pulse detector devices and determines a relative delay between the respective pathways based on timing the received indications, and includes a memory that stores the relative delay in association with an identification of the corresponding after-pulse detector devices.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to the application entitled “Method andApparatus for photosensor Gain and Scintillation Crystal OpticalCoupling Monitoring in Radiation Detectors” (U.S. application Ser. No.13/656,306), the contents of which are incorporated herein by reference.

FIELD

The embodiments described herein relate generally to a system and amethod of improving timing accuracy for a PET imaging system.

BACKGROUND

In PET imaging, or positron emission tomography, a radiopharmaceuticalagent is administered, via injection, inhalation and/or ingestion, to apatient. The physical and bio-molecular properties of the agent thenconcentrate at specific locations in the human body. The actual spatialdistribution, intensity of the point and/or region of accumulation, aswell as the kinetics of the process from administration and capture toeventual elimination, all have clinical significance. During thisprocess, the positron emitter attached to the radiopharmaceutical agentemits positrons according to the physical properties of the isotope,such as half-life, branching ratio, etc. Each positron interacts with anelectron of the object, is annihilated and produces two gamma rays at511 keV (electron-positron annihilation event), which travel atsubstantially 180 degrees apart. The two gamma rays then cause ascintillation event at a scintillation crystal of the PET detector,which detects the gamma rays thereby. By detecting these two gamma rays,and drawing a line between their locations or “line-of-response,” thelikely location of the original annihilation is determined. While thisprocess only identifies one line of possible interaction, accumulating alarge number of these lines, and through a tomographic reconstructionprocess, the original distribution is estimated with useful accuracy. Inaddition to the location of the two scintillation events, if accuratetiming—within a few hundred picoseconds—is available, time-of-flightcalculations are also made in order to add more information regardingthe likely position of the annihilation event along the line.Limitations in the timing resolution of a scanner determine the accuracyof the positioning along this line. Limitations in the determination ofthe location of the original scintillation events determine the ultimatespatial resolution of the scanner. A specific characteristic of theisotope (for example, energy of the positron) contributes (via positronrange and co-linearity of the two gamma rays) to the determination ofthe spatial resolution for a specific radiopharmaceutical agent.

As timing accuracy is an important factor, timing differences due toelectronic pathway disparities need to be considered and addressed. Theconventional way of measuring differences due to electronic pathwaydisparities includes using a signal generated from an external sourcesuch as a radioactive isotope. Thus, a radioactive source with phantomis typically used to measure the electronic pathway timing disparities.When processing the obtained data, an iterative method is used toperform measurements due to the indirect way in which the timing ismeasured. This iterative method is time consuming and costly.

The “background” description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent it is described in thisbackground section, as well as aspects of the description which may nototherwise qualify as prior art at the time of filing, are neitherexpressly or impliedly admitted as prior art against the presentembodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosed embodiments and many ofthe attendant advantages thereof will be readily obtained as the samebecomes better understood by reference to the following detaileddescription when considered in connection with the accompanyingdrawings, wherein:

FIG. 1 illustrates an example of an after pulse according to oneembodiment;

FIG. 2 illustrates an exemplary after-pulse detector circuit;

FIG. 3 illustrates a system configuration in which multiple zones aredesignated for the photosensor array;

FIG. 4 illustrates an exemplary timing spectrum of after-pulses from azone;

FIG. 5 illustrates another view of the system configuration in whichmultiple zones are designated for the photosensor array;

FIG. 6 illustrates a process flow of the system according to oneembodiment; and

FIG. 7 illustrates an exemplary computer system according to oneembodiment.

DETAILED DESCRIPTION

In one embodiment, there is described a system including a detectorincluding a photosensor that outputs a signal. Also included are aplurality of after-pulse detector devices independently connected to thephotosensor via respective electronic pathways. The after-pulse detectordevices each detect an after-pulse in the output signal and output anindication when the after-pulse is detected, the after-pulserepresenting an after-event in the photosensor triggered from a previousphoton producing event. Also included in the system is a processingdevice that receives the indication of the detection of the after-pulsefrom each of the plurality of after-pulse detector devices, and todetermine relative delays between the respective pathways based onrelative timing of the received indications. The system also includes amemory that stores each determined relative delay in association with anidentification of a corresponding after-pulse detector device.

In another embodiment, the system includes a timing calibrator thatcalibrates timing for the electronic pathways based on the determinedrelative delays.

In another embodiment, the after-pulse detector devices each include anintegrating filter that filters out signals other than the after-pulse.

In another embodiment, each after-pulse detector device does not detectan after-pulse when the filtered integrated signal is greater than 100keV.

In another embodiment, the detector includes a plurality ofphotosensors.

In another embodiment, the plurality of photosensors are assigned to aplurality of trigger zones.

In another embodiment, at least one photosensor is assigned to two zonesof the plurality of trigger zones.

In another embodiment, each of the plurality of after-pulse detectordevices is associated with a corresponding one of the plurality oftrigger zones.

Further, in another embodiment, there is described a timing delaydetection method. The method includes the steps of outputting a signalfrom a detector including a photosensor, detecting an after-pulse in theoutput signal at a plurality of after-pulse detector devicesindependently connected to the photosensor via respective electronicpathways, the after-pulse representing an after-event in the photosensortriggered from a previous photon producing event, outputting, at theplurality of after-pulse detector devices, an indication when theafter-pulse is detected, receiving, at a processing device, theindication of the detection of the after-pulse from each of theplurality of after-pulse detector devices, determining, at theprocessing device, relative delays between the respective pathways basedon relative timing of the received indications, and storing eachdetermined relative delay in association with an identification of acorresponding after-pulse detector device.

Further, in another embodiment, there is described an after-pulsedetector device independently connected to a photosensor via respectiveelectronic pathways. The after-pulse detector device includes anintegrator that receives a signal from the photosensor, that performsintegration on the received signal, and that outputs an integratedsignal. Also included in the after-pulse detector device is a firstcomparator that receives the signal from the photosensor and thatcompares the amplitude of the signal with a first reference value, and asecond comparator that receives the integrated signal from theintegrator and that compares the integrated signal with a secondreference value. The after-pulse detector device additionally includesan AND logic gate that receives a first output from the first comparatorand a second output from the second comparator and outputs an indicationof after-pulse detection in response to both the first output and thesecond output being logic level high.

Further, in another embodiment, there is described an after-pulsedetector device independently connected to a photosensor via respectiveelectronic pathways. The after-pulse detector device including an analogto digital (A/D) converter that receives a signal from the photosensorand that converts the signal into a digital signal. The after-pulsedetector further includes an integrating unit that receives the digitalsignal output from the A/D converter, performs integration on thedigital signal, and outputs an integrated signal, a first comparisonunit that receives the digital signal from the A/D converter and thatcompares an amplitude of the signal with a first reference value, and asecond comparison unit that receives the integrated signal from theintegrating unit and to compare the integrated signal with a secondreference value. The after-pulse detector device additionally includesan output unit that outputs an indication of after-pulse detection inresponse to the first comparison unit determining that the amplitude ofthe signal is greater than the first reference value and the secondcomparison unit determining that the integrated signal is less than thesecond reference value.

Referring now to the drawings wherein like reference numbers designateidentical or corresponding parts throughout the several views, FIG. 1shows an analog output signal from a photosensor, which could be a PMTor a Silicon photo multiplier (SiPM). Information about timingdifferences between photosensors caused by differences in the electricalpathways can be determined from after-pulses such as after-pulse 10shown in FIG. 1.

After-pulses are a type of noise that is often observed in PMTs andSiPMs. The after-pulses are an after-event which follows an event. Theafter-event may be produced by ion feedback for PMTs or hole andelectron trapper for SiPMs. The event may include an electron-positronannihilation event or any event which would produce enough photons suchthat the photons are detected. For example, the beta decay from aLutetium (Lu) background of a LYSO (Lu_(1.8)Y_(0.2)SiO₅(Ce)) crystal canalso trigger an after-pulse from a PMT.

In addition, signals produced by this after-event are spontaneous,occurring without the need to add an external light source. As is shownin FIG. 1, these pulses follow the main signal pulse after a delayperiod. One mechanism that can cause after-pulses is the emission oflight from the latter stages of a PMT that finds its way back into thephotosensor. These types of after-pulses follow shortly after the mainpulse 15. After-pulses are useful for determining timing at leastbecause these pulses have a sharp rising edge. After-pulses not onlyhave a sharp leading edge, but also have a short duration in timeresulting in a small integrated value.

The sharp leading edge and relative high amplitude provide good timinginformation by way of the after-pulse passing the threshold andgenerating a trigger signal. The small integrated value also provides asignature that enables the after-pulses to be isolated by filtering outother signals.

Another type of after-pulse is a dark pulse, which may be caused byimperfections in the PMTs. Small amounts of residual gas can be ionizedby the passage of electrons through the PMT. The positive ions that areformed move in the reverse direction and some return back to the photodetector. The dark pulses may show up well after the after-pulsesbecause the velocity of the positive ions is relatively low. Althoughdark pulses are not typically used to determine timing because of thesmall amplitude of these pulses, it is possible to also use dark pulsesto determine timing.

FIG. 2 illustrates an example of an after-pulse detector. This detectoris used to detect the presence of an after-pulse and to output anindication that the after-pulse has been detected.

In an alternate embodiment, the after-pulse detector can be triggered bythe detection of an electron-positron annihilation event by a differentevent detector that is designed to detect the event that precedes theafter-pulse. As a result of such a trigger, the after-pulse detector canavoid processing the signals resulting from the main event. Theafter-pulse detector is turned off after the after-pulse has beendetected.

In an additional alternate embodiment, the after-pulse detector isimplemented by a combination of software and hardware. In particular,the after-pulse detector obtains the analog signal by way of ananalog-to-digital (A/D) converter that samples the signal output fromthe photosensor. The sampled digital signal is then processed usingintegration and filtering to determine whether an after-pulse is presentin the signal. The digital implementation of the after-pulse detectoralso keeps track of the timing of each signal so that signals can becompared for timing differences.

The after-pulse detector device shown in FIG. 2 includes an integrator21 that receives the input signal and produces a time domain integrationvalue. The after-pulse signals have a small time domain integrationvalue compared to the main signal generated from the main eventutilizing ionization radiation. Thus, the output from the integrator 21is input into a comparator 22, which filters out the events that have anintegration value that is greater than or equal to a value Vref2 23. Forexample, Vref2 23 could be set to be a value higher than an integratedsignal equivalent of 100 keV (a predetermined threshold associated withan after-pulse). It should be noted that the value of Vref2 23 may becalibrated to better match the particular photosensor from which anafter-pulse is being detected.

In addition to filtering out signals which have an integration footprintgreater than an after-pulse, the detector also filters out all signalsthat have an amplitude that is not above a certain threshold. Forexample, the comparator 24 is set to compare the input signal with Vref125, which is set to be above the noise floor but lower than the averageamplitude of the after-pulses.

The output of the comparators 22 and 24 are input to AND gate 26. Ifboth signals are high, a signal is output indicating that an after-pulsehas been detected.

FIG. 3 shows an exemplary structure of the PET imaging system havingmultiple photosensors separated into different zones 1, 2 and 3. Eachzone includes photosensors that are also included in another zone. Forexample, FIG. 3 illustrates that zone 1 includes photosensors 31 and 32,which are also included in zone 2. The photosensors in zone 1 (includingphotosensors 31 and 32) are connected to zone 1 electronics 33. Thephotosensors in zone 2 (including photosensors 31 and 32) are connectedto zone 2 electronics 34. Thus, although the same event will be detectedby photosensors 31 and 32, this event could be recorded at differenttimes by zone electronics 33 and zone electronics 34 due to differencesin the electronic pathways (38 and 39) illustrated between photosensors31 and 32 and zone electronics 33 and 34, respectively. The electronicpathways can alternatively include optical elements.

Similar structure is provided for photosensors 36 and 37, which arelocated in zone 2 and zone 3. Photosensors 36 and 37 are connected tozone electronics 34 and zone electronics 35 via electronic pathways 39and 30, respectively.

Each zone electronics 33, 34, and 35 includes an independent after-pulsedetector such as the one shown in FIG. 2.

FIG. 4 illustrates a comparison between signals received by differentzone electronics. For instance, FIG. 4 illustrates the differencesbetween the detection of the same after-pulse by zone electronics 33 and34. For instance, a single after-pulse could be originated fromphotosensors 31 and/or 32, and the corresponding signal would then senddown separate electronic pathways to zone electronics 33 and 34. Zoneelectronics 33 may receive signal 41 approximately 150 ns after theevent, while zone electronics 34 receives signal 42 approximately 225 nsafter the event. Thus, the timing difference Δt between the electronicpathways is 75 ns.

The zone electronics 33 and 34 may be connected to a single photosensor(e.g. 31 or 32) that is on the zone boundary, that is situated in twozones and that is connected to two sets of zone electronics. As aresult, at least some, if not all, of the remaining photosensors in thetwo zones may not be connected to more than one zone electronics.

Alternatively, multiple photosensors 31 and 32 may each be connected tomore than one zone electronics (e.g. 33 and 34). In this case, thesignals from the photosensors are summed, delayed, or filtered to ensurethat the same signal representing the same after-pulse event is comparedat the two zone electronics.

The same after-pulse signals can utilize multiple signal pathways inelectronics and be recorded multiple times at different time points. Asis illustrated above, the timing differences can be measured by thetiming spectrum.

For example, in PET system design, photosensors may be shared betweenadjacent trigger zones. The same signal from one after-pulse can triggerneighboring trigger zones (e.g. zone 1 and zone 2, etc) and thus berecorded twice. The time difference between these two recorded signalsis a measurement of relative timing offset between two trigger zones.Because of the small time domain integration value, the spread(full-with-half-maximum) of the time difference measurement is also verysmall, as shown in FIG. 4. This direct method provides accurate relativetiming offset with fast computation.

Although this embodiment is not used to arrive at an absolute value oftiming offset from optics, it is nevertheless a very fast and efficientway to estimate relative timing offset for electronics. These timedifferences, due to differences in electronic paths, are usually anorder of magnitude higher than the actual time differences due tooptics. Using the relative timing offset as the initial setting beforerunning accurate timing test/calibration, significantly accelerates thecalibration process and improves the calibration accuracy.

FIG. 5 illustrates another example of the PET system according to oneembodiment. In this example, photosensor signals are sent to multiplezone electronics, and thus any relative time difference can be estimatedby analyzing the time differences between adjacent zones triggered bythe after-pulse. For example, zone 1 and zone 2 both includephotosensors 51 and 52 and zone 2 and zone 3 both include photosensors53 and 54. When zone electronics 1 or zone electronics 2 detect thesharp rising edge of the after-pulse, an indication signal is output.The timing differences between the outputs from zone electronics 1 andzone electronics 2 can be used to determine the relative time delaybetween photosensors 51 and 52 and zone 1 and zone 2 electronics. Theprocessing circuit 59 that determines the relative delay between theelectronic pathways is shown in FIG. 5. This processing circuit receivesthe indications from the various zone electronics 1, 2, and 3 andutilizes this information to determine the time delay difference. Forexample, the processing circuit 59 may receive the signals from the zoneelectronics that indicate that an after-pulse has been detected. Usingthese signals, the processing circuit 59 determines the time differencebetween receptions of the signals. Using this information, the relativedifference can be determined and recorded. In an alternative embodiment,the processing circuit 59 is designed such that each of the zoneelectronics 1, 2, and 3 is connected to the time delay processingcircuit 59 so that there is no delay difference between the respectivepathways.

FIG. 6 illustrates a flow diagram of the process of detectingafter-pulses and comparing the timings of the after-pulses detected bydifferent zone electronics.

In step S100, an after-pulse is originated by photosensors that are on azone boundary and are connected to two sets of zone electronics. Aphotosensor is associated with a zone when the photosensor is connectedto the particular electronics associated with the zone. The after-pulseis originated after the main event is originated.

In step S101, a signal is transmitted from photosensors to each zoneelectronics connected thereto. The photosensors continuously transmitsignals to the zone electronics. The after-pulse signal is originatedfrom photosensor after it detects a large enough amount of photons,which is usually from luminescence produced by interaction betweenradiation and scintillator.

In step S102, each respective signal is received by each of the zoneelectronics and each signal is analyzed to determine whether anafter-pulse is detected.

In step S103, in response to an after-pulse being detected, a signal isoutput for each zone electronics that detects the after-pulse.

In step S104, the respective time stamp of the output signal from eachof the two zone electronics that are connected to the same group ofphotosensors are compared for the particular after-pulse that isdetected by the group of photosensors.

In step S105, the relative difference between the two outputs isrecorded.

In step S106, the recorded relative difference is input into acorrection algorithm that is used to correct time differences.

In step S107, the relative difference between the two outputs istransmitted to detection electronics, which adjust the system timingoffset calibration used for detecting an event.

Certain portions of the processing can be implemented using some form ofcomputer processor. As one of ordinary skill in the art would recognize,the computer processor can be implemented as discrete logic gates, as anApplication Specific Integrated Circuit (ASIC), a Field ProgrammableGate Array (FPGA) or other Complex Programmable Logic Device (CPLD). AnFPGA or CPLD implementation may be coded in VHDL, Verilog or any otherhardware description language and the code may be stored in anelectronic memory directly within the FPGA or CPLD, or as a separateelectronic memory. Further, the electronic memory may be non-volatile,such as ROM, EPROM, EEPROM or FLASH memory. The electronic memory mayalso be volatile, such as static or dynamic RAM, and a processor, suchas a microcontroller or microprocessor, may be provided to manage theelectronic memory as well as the interaction between the FPGA or CPLDand the electronic memory.

Alternatively, the computer processor may execute a computer programincluding a set of computer-readable instructions that perform thefunctions described herein, the program being stored in any of theabove-described non-transitory electronic memories and/or a hard diskdrive, CD, DVD, FLASH drive or any other known storage media. Further,the computer-readable instructions may be provided as a utilityapplication, background daemon, or component of an operating system, orcombination thereof, executing in conjunction with a processor, such asa Xenon processor from Intel of America or an Opteron processor from AMDof America and an operating system, such as Microsoft VISTA, UNIX,Solaris, LINUX, Apple, MAC-OSX and other operating systems known tothose skilled in the art.

In addition, certain features of the embodiments can be implementedusing a computer based system 1000 shown in FIG. 7. The computer 1000includes a bus B or other communication mechanism for communicatinginformation, and a processor/CPU 1004 coupled with the bus B forprocessing the information. The computer 1000 also includes a mainmemory/memory unit 1003, such as a random access memory (RAM) or otherdynamic storage device (e.g., dynamic RAM (DRAM), static RAM (SRAM), andsynchronous DRAM (SDRAM)), coupled to the bus B for storing informationand instructions to be executed by processor/CPU 1004. In addition, thememory unit 1003 may be used for storing temporary variables or otherintermediate information during the execution of instructions by the CPU1004. The computer 1000 may also further include a read only memory(ROM) or other static storage device (e.g., programmable ROM (PROM),erasable PROM (EPROM), and electrically erasable PROM (EEPROM)) coupledto the bus B for storing static information and instructions for the CPU1004.

The computer 1000 may also include a disk controller coupled to the busB to control one or more storage devices for storing information andinstructions, such as mass storage 1002, and drive device 1006 (e.g.,floppy disk drive, read-only compact disc drive, read/write compact discdrive, compact disc jukebox, tape drive, and removable magneto-opticaldrive). The storage devices may be added to the computer 1000 using anappropriate device interface (e.g., small computer system interface(SCSI), integrated device electronics (IDE), enhanced-IDE (E-IDE),direct memory access (DMA), or ultra-DMA).

The computer 1000 may also include special purpose logic devices (e.g.,application specific integrated circuits (ASICs)) or configurable logicdevices (e.g., simple programmable logic devices (SPLDs), complexprogrammable logic devices (CPLDs), and field programmable gate arrays(FPGAs)).

The computer 1000 may also include a display controller coupled to thebus B to control a display, such as a cathode ray tube (CRT), fordisplaying information to a computer user. The computer system includesinput devices, such as a keyboard and a pointing device, for interactingwith a computer user and providing information to the processor. Thepointing device, for example, may be a mouse, a trackball, or a pointingstick for communicating direction information and command selections tothe processor and for controlling cursor movement on the display. Inaddition, a printer may provide printed listings of data stored and/orgenerated by the computer system.

The computer 1000 performs at least a portion of the processing steps inresponse to the CPU 1004 executing one or more sequences of one or moreinstructions contained in a memory, such as the memory unit 1003. Suchinstructions may be read into the memory unit from another computerreadable medium, such as the mass storage 1002 or a removable media1001. One or more processors in a multi-processing arrangement may alsobe employed to execute the sequences of instructions contained in memoryunit 1003. In alternative embodiments, hard-wired circuitry may be usedin place of or in combination with software instructions. Thus,embodiments are not limited to any specific combination of hardwarecircuitry and software.

As stated above, the computer 1000 includes at least one computerreadable medium 1001 or memory for holding instructions programmedaccording to the teachings of the embodiments and for containing datastructures, tables, records, or other data described herein. Examples ofcomputer readable media are compact discs, hard disks, floppy disks,tape, magneto-optical disks, PROMs (EPROM, EEPROM, flash EPROM), DRAM,SRAM, SDRAM, or any other magnetic medium, compact discs (e.g., CD-ROM),or any other medium from which a computer can read.

Stored on any one or on a combination of computer readable media, thepresent embodiments includes software for controlling the mainprocessing unit 1004, for driving a device or devices for implementingthe embodiments, and for enabling the main processing unit 1004 tointeract with a human user. Such software may include, but is notlimited to, device drivers, operating systems, development tools, andapplications software. Such computer readable media further includes thecomputer program product of the present embodiments for performing allor a portion (if processing is distributed) of the processing performedin implementing the embodiments.

The computer code elements on the medium of the present embodiments maybe any interpretable or executable code mechanism, including but notlimited to scripts, interpretable programs, dynamic link libraries(DLLs), Java classes, and complete executable programs. Moreover, partsof the processing of the present embodiments may be distributed forbetter performance, reliability, and/or cost.

The term “computer readable medium” as used herein refers to any mediumthat participates in providing instructions to the CPU 1004 forexecution. A computer readable medium may take many forms, including butnot limited to, non-volatile media, and volatile media. Non-volatilemedia includes, for example, optical, magnetic disks, andmagneto-optical disks, such as the mass storage 1002 or the removablemedia 1001. Volatile media includes dynamic memory, such as the memoryunit 1003.

Various forms of computer readable media may be involved in carrying outone or more sequences of one or more instructions to the CPU 1004 forexecution. For example, the instructions may initially be carried on amagnetic disk of a remote computer. An input coupled to the bus B canreceive the data and place the data on the bus B. The bus B carries thedata to the memory unit 1003, from which the CPU 1004 retrieves andexecutes the instructions. The instructions received by the memory unit1003 may optionally be stored on mass storage 1002 either before orafter execution by the CPU 1004.

The computer 1000 also includes a communication interface 1005 coupledto the bus B. The communication interface 1004 provides a two-way datacommunication coupling to a network that is connected to, for example, alocal area network (LAN), or to another communications network such asthe Internet. For example, the communication interface 1005 may be anetwork interface card to attach to any packet switched LAN. As anotherexample, the communication interface 1005 may be an asymmetrical digitalsubscriber line (ADSL) card, an integrated services digital network(ISDN) card or a modem to provide a data communication connection to acorresponding type of communications line. Wireless links may also beimplemented. In any such implementation, the communication interface1005 sends and receives electrical, electromagnetic or optical signalsthat carry digital data streams representing various types ofinformation.

The network typically provides data communication through one or morenetworks to other data devices. For example, the network may provide aconnection to another computer through a local network (e.g., a LAN) orthrough equipment operated by a service provider, which providescommunication services through a communications network. The localnetwork and the communications network use, for example, electrical,electromagnetic, or optical signals that carry digital data streams, andthe associated physical layer (e.g., CAT 5 cable, coaxial cable, opticalfiber, etc). Moreover, the network may provide a connection to a mobiledevice such as a personal digital assistant (PDA) laptop computer, orcellular telephone.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the embodiments. Indeed the novel methods and systems describedherein may be embodied in a variety of other forms; furthermore, variousomissions, substitutions, and changes in the form of the methods andsystems described herein may be made without departing from the spiritof the embodiments. The accompanying claims and their equivalents areintended to cover such forms or modifications as would fall within thescope and spirit of the embodiments.

The invention claimed is:
 1. A timing delay detection system,comprising: a detector including a photosensor configured to output asignal; a plurality of after-pulse detector devices independentlyconnected to the photosensor via respective electronic pathways, theafter-pulse detector devices each configured to detect an after-pulse inthe output signal and output an indication when the after-pulse isdetected, the after-pulse representing an after-event in the photosensortriggered from a previous photon producing event; a processing deviceconfigured to receive the indication of the detection of the after-pulsefrom each of the plurality of after-pulse detector devices, and todetermine relative delays between the respective pathways based onrelative timing of the received indications; and a memory configured tostore each determined relative delay in association with anidentification of a corresponding after-pulse detector device.
 2. Thetiming delay detection system according to claim 1, further comprising:a timing calibrator configured to calibrate timing for the electronicpathways based on the determined relative delays.
 3. The timing delaydetection system according to claim 1, wherein the after-pulse detectordevices each include an integrating filter configured to filter outsignals other than the after-pulse.
 4. The timing delay detection systemaccording to claim 3, wherein each after-pulse detector device isconfigured to not; detect an after-pulse when the filtered integratedsignal is greater than 100 keV.
 5. The timing delay detection systemaccording to claim 1, wherein the detector includes a plurality ofphotosensors.
 6. The timing delay detection system according to claim 5,wherein the plurality of photosensors are assigned to a plurality oftrigger zones.
 7. The timing delay detection system according to claim6, wherein at least one of the photosensors is assigned to two zones ofthe plurality of trigger zones.
 8. The timing delay detection systemaccording to claim 6, wherein each of the plurality of after-pulsedetector devices is associated with a corresponding one of the pluralityof trigger zones.
 9. A timing delay detection method, comprising:outputting a signal from a detector that includes a photosensor;detecting an after-pulse in the output signal at a plurality ofafter-pulse detector devices independently connected to the photosensorvia respective electronic pathways, the after-pulse representing anafter-event in the photosensor triggered from a previous photonproducing event; outputting, at the plurality of after-pulse detectordevices, an indication when the after-pulse is detected; receiving, at aprocessing device, the indication of the detection of the after-pulsefrom each of the plurality of after-pulse detector devices; determining,at the processing device, relative delays between the respectivepathways based on relative timing of the received indications; andstoring each determined relative delay in association with anidentification of a corresponding after-pulse detector device.
 10. Thetiming delay detection method according to claim 9, further comprising:performing a timing calibration process based on the determined relativedelays.
 11. The timing delay detection method according to claim 9,further comprising filtering out signals other than the after-pulse withan integrating filter.
 12. The timing delay detection method accordingto claim 11, wherein the detecting step further comprises not detectingan after-pulse when the filtered integrated signal is greater than 100keV.
 13. The timing delay detection method according to claim 9, whereinthe outputting further comprises outputting the signal from the detectorthat includes a plurality of photosensors.
 14. The timing delaydetection method according to claim 13, wherein the outputting furthercomprises outputting the signal from the detector that includes theplurality of photosensors assigned to a plurality of trigger zones. 15.The timing delay detection method according to claim 14, wherein theoutputting further comprises outputting the signal from the detectorthat includes at least one photosensor assigned to two zones of theplurality of trigger zones.
 16. The timing delay detection methodaccording to claim 14, wherein the detecting further comprises detectingthe after-pulse in the output signal at the plurality of after-pulsedetector devices each associated with a corresponding one of theplurality of trigger zones.